The present invention relates to an extruder, and more particularly to an extruder of the degassing and modification type in which the degassing, desolventation and modification of synthetic resins are directly performed.
One of the problems of the present processes is that in conventional polyethylene, issued from a polymerization apparatus, ethylene monomer, etc. are contained, which are unreacted in the polymerization process, that is, it has the danger of exploding, etc. To transport products containing ethylene monomer, etc. is legally prohibited in terms of safety. Therefore, in order to remove ethylene monomer, etc. a method has been conventionally carried out wherein pellets of conventional polyethylene are received in a tank and blown with hot air or inactive gas from the bottom so that the ethylene monomer, etc. contained in the pellets are discharged out of the tank accompanied by the hot air. However, these discharged poisonous gasses such as ethylene monomer, etc. are diffused in the air and endanger the health of residents living around factories which manufacture synthetic resins, and at present this is becoming a serious environmental problem as regards pollution. Further, conventional polyethylene is required to be modified, principally its optical properties, in order to improve its solid state properties.
On the other hand, in the manufacturing of synthetic rubber, as a method for separation of hexane from the synthetic rubber which hexane has been used as a solvent in the polymerization process, water displacement method has been applied, however, this method usually uses very large quantities of water.
On applying this method, hexane goes into water to be separated from synthetic rubber together with the water, which results in the generation of water containing hexane, so from the view point of pollution it cannot be directly discharged into sewerage or the like. This necessiates a treatment installation therefor.
In order to cope with these problems caused by the present processes, an apparatus is required that can simultaneously carry out a pelletizing, degassing, desolvention and modification of synthetic resins. An apparatus which can effectively perform these operations by a single device a single- or double-screw type extruder has been hitherto proposed.
The conventional single-screw type extruder has very few mechanical problems, however, it cannot sufficiently degass, desolvenize or modify material. On the other hand, a conventional double-screw type extruder has various problems of a mechanical nature which will be described below. If it is constituted in such a manner that both screws are made to mesh with each other or both screws are made not to mesh, but made to have flights as far as their forward ends, the screw thrust load due to pressure from molten resin at the forward ends of the screws is applied uniformly to both screws, but, in order to have a bearing construction to resist the screw thrust load the bearings are required to have a sufficiently large outer diameter, whereas, in the case of the double-screw type extruder, since the distance between the axes of both screws is limited to a relatively small value, it is impossible to provide bearings having a large outer diameter to support the forward ends of the screws. Thus, in the double-screw type extruder it is impossible to have a construction to receive a large thrust load as compatible with that in the single-screw type extruder. Further, at a portion where the two screws are circumscribed with each other, since a hollow space is created from the necessity of machining, backflow of the molten resin is made large so that resin pressure cannot be generated at the forward portions of the screws.
Thus, it will be appreciated that since the conventional methods for degassing or desolvention pollute a natural environment such as atmosphere, water, etc. they are subjected to regulations concerning the prevention of pollution and extruders which have been conventionally adopted as means for obviating the problems with pollution still have various drawbacks.